Ultrasound systems have become widely-used diagnostic tools for various medical applications. Many ultrasound systems, compared to some other diagnostic tools or systems, are non-invasive and non-destructive. An ultrasound system generally includes a probe for approaching or placing directly on and moving over a subject, such as a patient. The ultrasound system may provide visualization of the subject's internal structures, such as tissues, vessels, and/or organs. The ultrasound system works by electrically-exciting transducer elements inside the probe to generate ultrasound signals, which travel into the body, and by receiving the echo signals reflected from tissues, vessels, and/or organs. The reflected echo signals are then processed to produce a visualization of the subject's internal structures.
One of the applications of ultrasound systems is for measuring blood flow velocity. Such information can be useful in cardiovascular studies and other medical areas. Several methods have been developed to present different aspects of blood flow, such as B-mode imaging and color Doppler imaging. B-mode imaging of the flow field is often used to locate vessels, to measure their size, and to observe flow structure. The B-mode image displays the brightness indicating the intensities of the ultrasound signals reflected from the target object. In addition to the grayscale display, flow velocity may be rendered in color Doppler imaging as an overlay of the B-mode image to display the measurement of blood cells velocity within a blood vessel. In conventional systems, the color Doppler image places several limitations to the quality and effectiveness of such systems.
First, the frame rate of the color Doppler imaging could be low. B-mode image can be done at a relatively high frame rate since only one transmit pulse is needed for each image line of the display. In contrast, each Doppler image line needs to be interrogated a number of times in order to estimate a Doppler shift at various points along the line. Each interrogation along the line acquires a full line of echo data, and the set of samples acquired over time for each point on the image line is referred to as an ensemble. Since the interrogation in color Doppler imaging takes time, people often sacrifices B-mode image qualities, such as less line densities, less sampling rate, and/or multi beamformers, in order to obtain a useful frame rate.
Second, conventional systems and methods may have poor lateral resolution. For example, in a conventional color Doppler imaging system 100 as shown in FIG. 1A, conventional methods often use multiple transducer elements 112 on a probe 110 to transmit ultrasound pulses toward a focus point. Such conventional methods are used in order to obtain desired signal-to-noise ratio and improve the lateral resolution at the focus point. While the focus point has the highest lateral resolution, the rest of the beam suffers from poor lateral resolution.
Third, the conventional system 100 as shown in FIG. 1A also suffers from a pre-drawn window limitation. In order to obtain a detailed quantification of flow velocity, a much smaller sample volume needs to be chosen ahead of time within a region of interest. A sonographer often needs to draw a window in the scan area, only in that window, the color Doppler method is used such as repeated transmission of pluses, and results only shows in that window. This pre-drawn window requirement means that moving reflectors that lie outside of the pre-defined window may not be identified until a separate Doppler imaging session is conducted. A full-scan area of color Doppler image is therefore difficult to obtain.
Fourth, using conventional methods and systems, it is difficult to obtain both the absolute velocity and direction. The system shown in FIG. 1A may require direction steering and multiple transmissions to detect the velocity of a moving reflector. In recent year, plane wave as shown in FIGS. 1B-1D has drawn attention in the industry. Due to plane wave's lack of lateral resolution, relatively to the conventional methods and systems shown in FIG. 1A, a great scale of receive beams may be formed simultaneously. However, the greater scale and without pre-drawn window limitation may increase frame rates while trading off lateral resolution. In order to obtain lateral information, similar to the conventional method shown in FIG. 1A, direction steering may be necessary to perform angled transmission during velocity detection.
The angled transmission is due to the Doppler effect of the blood cell velocity relative to the direction of the incoming ultrasound direction. Flow that is transverse to the incoming ultrasound direction is not detectable using conventional methods including the plane wave methods. The amplitude component of a velocity vector obtained using conventional methods represents only the component of the flow velocity vector that lies along the transmit/receive scan line axis. For lateral blood vessels, in order to obtain the amplitude component of the velocity vector, the plane wave method would have to change the transmit ultrasound direction from perpendicular to an angled direction.
For example, FIGS. 1B-1D illustrate conventional plane wave methods and systems of obtaining the direction and the amplitude of the two-dimensional (2D) blood flow velocity. The conventional systems may include a probe 110 with a plurality of transmit transducer elements 112. During one transmit event 150, the amplitude component of the 2D velocity vector within a blood vessel 130 may be detected, since velocity of blood cells within the blood vessel 130 relative to the direction of the incoming ultrasound direction is not zero, i.e. point C 132 within the blood vessel 130 has a non-zero velocity relative to the direction of the incoming ultrasound. However, in another blood vessel 130, which is transverse to the incoming ultrasound direction, the velocity cannot be detected in the transmit event 150, i.e. point A 122 and point B 124 within the blood vessel 120 have zero velocity relative to the direction of the incoming ultrasound in FIG. 1B.
In order to detect the 2D blood flow velocity in the blood vessel 130, another transmit event 180 as illustrated in FIG. 1C has to be performed to change the transmit ultrasound direction from perpendicular to the blood vessel 130 to an angled direction. In FIG. 1C, point B 124 has non-zero velocity relative to the angled direction of the incoming ultrasound. However, the plane wave in FIG. 1C would miss point A 122. In order to calculate point A 122, another angled plane wave transmit event 190 as shown in FIG. 1D may have to be performed. Nonetheless, in the second angled plane wave transmission 190, point C 126 velocity cannot be detected.
As shown in FIGS. 1B-1D, the plane wave methods of direction steering and multiple rounds of transmissions of signals from different angles are cumbersome and inefficient. After three rounds of transmission in FIGS. 1B-1D from three different angles, the velocity at point A 122 may only be detected once in the transmission event 190 of FIG. 1D, the velocity at point B 124 may only be detected twice in the transmission events 180 of FIG. 1C and 190 of FIG. 1D, and the velocity at point C 126 may only be detected twice in the transmission events 150 of FIG. 1B and 180 of FIG. 1C. Thus, after three rounds of transmission, each point A 122, B 124, and C 126 has less than three views to contribute to the computation of 2D velocity vector. Therefore, due to the complex nature of direction steering, the plane wave methods have uncertainties in cross-correlation estimates of the amplitude and angle of 2D velocity vector.
There is a need, therefore, for a fast and simple full-scan area Doppler imaging method and system capable of obtaining both absolute velocity and direction of blood flow with comparable frame rate as B-mode imaging.